The present invention relates to a centrally operated locking device for the doors of a vehicle.
Locking systems of the type to which the invention pertains usually include a locking drive for each door and a central control and command unit for all of these drives. Each drive should be constructed with a manual override. Our U.S. Pat. No. 4,135,377 discloses such a system wherein particularly each drive unit includes a reversible motor for driving a worm gear, and a travelling nut runs on that gear in axial direction as the worm rotates. This nut, in turn, is coupled to the door lock actuator.
Our patent discloses specifically switches for controlling and limiting the travelling range of the nut on the worm. These switches turn the motor off in order to avoid thermal overload when the nut hits a stop. The motor is additionally protected by a temperature-sensitive element, should the switches fail. Without protection in one form or another, the motor will stop whenever the nut hits an impediment, but the current will drastically increase unless turned off.
It has now been discovered that in the case of switch failure, the nut may, for example, be driven against stop surfaces in such a manner that strong friction occurs between the nut and the surface. For example, in our patent, the nut can be driven against and onto the unthreaded portion of the worm spindle and strong friction will hold it. Upon reversing the motor, the friction may be too strong so that rotating the spindle will not pry the nut loose. It should be noted that this problem is unrelated to the overload clutch which is the specific subject of that patent. The problem presently alluded to occurs only in the case of equipment failure.